This invention relates to encapsulated electrical assemblies and to a product which is formed thereby.
Encapsulated electrical assemblies are well known to the art, wherein a plurality of electrical components including, for example, series-connected diodes, are encapsulated in a thermally conductive mass. Present arrangements, however, do not permit sufficient cooling of a large number of semiconductor devices which are compactly contained within a given volume to define a high voltage assembly. In particular, present arrangements do not permit the use of known highly reliable semiconductors connected in series in a compact assembly while preserving the established reliability rating of the components. In order to accomplish this, it is necessary to provide a superior thermal path for junction cooling, while providing dielectric isolation for the high voltage applications. Thus, known techniques fail to produce a product capable of operation at base plate temperatures of 125.degree. C. and do not use established reliability components. Those products which are operated at high temperatures and which use established reliability components are not capable of operation at high voltages with fast recovery times.
Among known techniques are those in which the silicon junctions are stacked in series to achieve the necessary high breakdown voltage. This produces an assembly with relatively poor temperature distribution characteristics since the thermal conductivity of silicon is relatively poor. Therefore, the central junctions or chips become relatively hot and tend to go into thermal avalanche. When the central chips go into thermal avalanche, the adjacent chips will also go into thermal avalanche and the entire assembly begins to fail. This failure mechanism is accelerated by the degradation of reverse recovery time with temperature which further degrades the junctions. Finally, the structure referred to above normally employs an enclosing polymer shell which has poor thermal conductivity to surrounding areas.